Secreted proteins play an integral role in the formation, differentiation, and maintenance of cells in multicellular organisms. For instance, secretory proteins are known in the art to be involved in signaling between cells which are not in direct contact. Such secreted signaling molecules are particularly important in the development of vertebrate tissue during embryogenesis as well as in the maintenance of the differentiated state of adult tissues. For example, inductive interactions that occur between neighboring cell layers and tissues in the developing embryo are largely dependent on the existence and regulation of secreted signaling molecules. In inductive interactions, biochemical signals secreted by one cell population influence the developmental fate of a second cell population, typically by altering the fate of the second cell population. For example, the Wnt proteins are now recognized as one of the major families of developmentally important signaling molecules in organisms ranging from Drosophila to mice.
The Wnt gene family encode a large class of secreted proteins related to the Int1/Wnt1 proto-oncogene and Drosophila wingless (“Wg”), a Drosophila Wnt1 homologue, (Cadigan et al. (1997) Genes & Development 11:3286–3305). Wnts are expressed in a variety of tissues and organs and are required for many developmental processes, including segmentation in Drosophila, endoderm development in Caenorhabditis elegans, establishment of limb polarity, neural crest differentiation, kidney morphogenesis, sex determination, and brain development in mammals (reviewed in Parr and McMahon (1994) Curr. Opinion Genetics & Devel. 4:523–528; Cadigan and Nusse, supra).
Recent studies in diverse organisms have led to identification of several components of the Wnt signal transduction pathway in responding cells (Cadigan and Nusse, supra). Wnt signals are transduced by the Frizzled (“Fz”) family of seven transmembrane domain receptors (Bhanot et al. (1996) Nature 382:225–230). The resulting signal leads to the activation of the cytoplasmic protein Dishevelled (Dsh) and stabilization of Armadillo/β-catenin (Perrimon (1994) Cell 76:781–784). Negative regulators of the Wnt pathway include glycogen synthase kinase 3 (GSK3)/shaggy (Perrimon, supra), the tumor suppressor gene product adenomatous polyposis coli (APC) (Gumbiner (1997) Curr. Biol. 7:R443–436) and a novel protein, called Axin (Zeng et al. (1997) Cell 90:181–192). In the absence of a Wnt ligand, these proteins promote phosphorylation and then degradation of β-catenin, whereas Wnt signaling inactivates GSK3, thus preventing β-catenin degradation. As a result, β-catenin is translocated to the nucleus, where it forms a complex with TCF transcription factors and activates target gene expression (Cadigan and Nusse, supra). Deregulation of this pathway can lead to carcinogenesis (reviewed by Gumbiner, supra), emphasizing the long-recognized connection between Wnts, normal development and cancer. This connection has been further established recently with the identification the c-Myc protooncogene as a target of Wnt signaling (He et al. (1998) Science 281:1509–3512).
While the outcome of Wnt signaling may be influenced by multiple intracellular regulatory mechanisms, recent studies have identified several classes of secreted factors which can modulate Wnt action outside of the cell. These include Cerberus, a secreted Wnt inhibitor implicated in head development (Bouwmeester et al (1996) Nature 382:595–601), and a family of proteins related to the extracellular domain of Frizzled. These Frizzled-related proteins (“FRPs”) (Rattner et al. (1997) Proc. Natl. Acad. Sci. USA 94:2859–2863), also known as secreted apoptosis-related proteins (“SARPs”), are encoded by several independently discovered genes including FrzA/FRP1, SDF5/FRP2, FrzB/FRP3, FRP4 and Sizzled (Melkonyan et al. (1997) Proc. Natl. Acad. Sci. USA 94:13636–13641; Finch et al. (1997) Proc. Natl. Acad. Sci. USA 94:6770–6775; Wang et al. (1997) Cell 88:747–766; Leyns et al. (1997) Cell 88:747–756; Mayr et al. (1997) Mech. Dev. 63:109–325; and Salic et al. (1997) Development 124:4739–4748). These proteins inhibit the ability of Xwnt8 to induce a secondary axis in frog embryos (for review see Zorn (1997) Curr. Biol. 7:R501–504), and are thought to compete for binding of Wnt ligands to the Frizzled receptors. Data on binding of certain FRPs to Xwnt8 (Wang et al., (1997) Biochem. Biophys. Res. Comm. 236:502–504; and Leyns et al., supra) and Wg corroborate this notion (Rattner et al., supra).
It is now recognized that many of these families of signaling molecules have a dual role to play in both the development of an organism as well as in promoting or maintaining the differentiated state of tissues in the adult animal. Furthermore, major families of signaling molecules have been implicated in controlling proliferation of cells in mature adult tissue, for example, during normal cell turnover in the adult organism as well as in tissue regeneration activated as a result of damage to the adult tissue. Given the important role of these signalling molecules such as the Wnts and FRPs in both developing and adult tissues, there exists a need for identifying novel modulators of such molecules for use in regulating a variety of cellular processes.